The present invention relates to a technology that prevents rotation irregularities of an image carrier in an image forming apparatus and to a technology that prevents color registration failure when toner images of respective colors formed on a plurality of image carriers are transferred onto a medium to be transferred, such as an intermediate transfer body in a color image forming apparatus.
The color image forming apparatus, which has the image carriers for respective primary colors yellow (Y), magenta (M), cyan (C), and black (K), forms toner images of the colors on the image carriers respectively, superimposes the formed toner images on an intermediate transfer body, and transfers the toner image with the four colors superimposed thereon on a transfer paper to form a color image.
The image carriers respectively configure image forming sections each equipped with an electrostatic charger, a developing device, a transfer device, a cleaning device and other related components, and the image forming sections of the respective colors and the intermediate transfer body are integrally formed to configure a process cartridge. This process cartridge can be pulled out from a main body of the image forming apparatus, and each of the image carrier units and developing devices and the like are configured to be able to be attached and separated relative to the process cartridge in the state in which the process cartridge is pulled out.
The toner images of the respective colors formed in the image forming sections of the respective colors are sequentially transferred onto the rotating intermediate transfer body by the transfer devices to form a synthesized color image. Creating a color image without color registration failure requires the toner images to be sequentially matched to the previous toner image and superimposed on the next toner image.
Because of the object as described above, the linear velocities of the image carriers and the medium to be transferred, such as the intermediate transfer body, need to be identical to each other with a high degree of precision. A difference in the linear velocities will cause color registration failure or image registration failure.
Driving of the image carriers is given by drive units provided in each of the image carriers. The drive unit transmits rotation which has been slowed down via a plurality of gears from a main motor to a rotary shaft. The rotary shaft can be inserted and removed through the cylindrical image carrier and is supported by a shaft bearing in the opposite side to rotate the image carrier.
The image carrier, which receives the rotation from the main motor via a gear train constituted by the plurality of gears as described above and is required for precision in all of the gears, generates slight rotation irregularities due to the influence of tolerances accumulated in each of the gears. Also, the intermediate transfer body has rotation irregularities because it is driven by the similar drive section. Thus, even though the linear velocities of the image carrier and the intermediate transfer body are matched, when the rotation irregularities occur in either, the other cannot follow the former, resulting in a blurred image.
On the other hand, in Patent Document 1 (Japanese Patent Publication Laid-Open No. 2000-112194), the transmission of the rotation from a rotary shaft to an image carrier is directly linked to a large gear connected via a fitting member, and a shaft bearing thereof having a coupling form into which a fitting section of the fitting member fits. It claims that with such a configuration, the rotation may be slowed down with the one large gear from a main motor to the image carrier, allowing the reduction of the number of gear teeth to decrease the influence of the accumulated tolerances and improve the rotation irregularities, so that the image carrier can follow the rotation irregularities of an intermediate transfer body.
However, the rotation irregularities could be improved but not be sufficiently small, and the image carrier could not follow the rotation irregularities of the intermediate transfer body and image defects sometimes occurred in the color image forming apparatus.
Further, when connecting the image carrier and the drive section with couplings, a state in which an image-carrier side coupling and a drive section side coupling abut each other at only one place (wherein referred to as one-point abutment) occurs, causing the generation of the rotation irregularities. The rotation irregularities due to this one-point abutment may be a factor that is preventing the image carrier from following the surface velocity of the intermediate transfer body.
To cope with the above problem, the applicant proposes, in Unexamined Japanese Patent Application No. 2005-62806 (corresponding to Japanese Patent Application No. 2004-43817), a configuration that provides play in a connection section of the couplings so that the couplings can be abutted at several places due to this play. With this configuration, the one-point abutment can be prevented with considerable probability.